Here are just a few of the (more realistic) possibilities that your fellow anonymous internet contributors have suggested:
You or someone else has accidentally set a "wake-up" timer for your TV.
Your remote batteries are low. Remotes are known to send off some weird signals when they're on the last bit of juice. Your TV could be interpreting infrared noise from your remote as an "On" command.
The power button on the remote or the TV itself could be stuck.
Somebody could be playing a prank on you.
Your neighbors could be accidentally powering on your TV through the window, especially if they have the same model synced up with a similar universal remote (like the one your cable company gives you).
Your pet might have just sat or stepped on your remote control while you were in the other room.
Perhaps it wasn't powered off entirely before (maybe you shut off the cable box but not the TV screen?), and it's now flickering back on from a power surge.
A fluorescent lamp inside the same room as the TV is blinking red and causing a sensitive infrared remote sensor to turn the set on and off.
It could just be an internal electrical malfunction, like a resistor that's changed value or a defective infrared sensor.